The present invention relates to an electric power tool provided with a motor which rotationally drives a rotational shaft which mounts a tool element.
An electric power tool of so-called electronic clutch type has been known. This type of electric power tool is configured to stop driving of a motor when rotational torque of a rotational shaft which mounts a tool element such as a driver bit exceeds a predetermined set torque.
This type of electric power tool is usually configured to set a current threshold in accordance with the set torque and, when a motor current reaches the current threshold, determines that the rotational torque of the rotational shaft has reached the set torque and stop driving of the motor.
The motor current Im can be described as below when V is a power supply voltage, E is a back-electromotive force generated upon driving the motor, and R is a resistance of an armature winding.Im=(V−E)/R 
When the motor starts driving, the back-electromotive force E=0, since a rotor is in a stationary state.
As a result, immediately after the motor starts driving, the motor current temporarily exceeds the current threshold. If the motor current is limited to be equal to or lower than the current threshold from immediately after the motor starts driving, driving of the motor cannot be continued.
For example, the electric power tool of electronic clutch type disclosed in Japanese Unexamined Patent Application Publication No. 2006′281404 is configured not to stop driving of the motor even if the motor current exceeds the current threshold unless such state continues for a predetermined period of time or more.
In other words, after the motor starts driving, the function of electronic clutch is stopped for a fixed time, so that the driving of the motor can continue.